A
Nimbostratus cloud is characterized by a formless
cloudA cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae...
layer that is almost uniformly dark gray. "Nimbo" is from the Latin word "nimbus", meaning rain. It is a stratiform cloud that produces rain, developing
cloud baseThis article refers to meteorology, for the airborne base of Captain Scarlet see Cloudbase.The cloud base is the lowest altitude of the visible portion of the cloud...
s between the surface and 10000 ft (3000 m). Nimbostratus usually has a thickness of 2000 meters. In rare cases, Nimbostratus can be very thin and accompanied by a separate layer of
altostratusAltostratus is a cloud belonging to a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. The sun can be seen shining through them, and they frequently cover the whole sky...
divided by a cloudless layer. Though found worldwide, nimbostratus is found more commonly in the middle latitudes.
Appearance
The base of a nimbostratus
baseThis article refers to meteorology, for the airborne base of Captain Scarlet see Cloudbase.The cloud base is the lowest altitude of the visible portion of the cloud...
cloud is dimmed by precipitation and is usually not clearly visible. In all cases, nimbostratus is accompanied by
pannusFractus clouds are small, ragged cloud fragments which, usually found under an ambient cloud base, form or have broken off a larger cloud, and are generally sheared by and shredded-looking due to strong winds. Fractus have irregular patterns, appearing much like torn pieces of cotton candy, change...
clouds, which develop underneath of nimbostratus. If the pannus layer is completely opaque, the presence of precipitation indicates presence of nimbostratus. The pannus movement is slow and uniform under nimbostratus.
Distinguishing features
Nimbostratus,
stratusA stratus cloud is a cloud belonging to a class characterized by horizontal layering with a uniform base, as opposed to convective clouds that are as tall or taller than wide . More specifically, the term stratus is used to describe flat, hazy, featureless clouds of low altitude varying in color...
,
altostratusAltostratus is a cloud belonging to a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. The sun can be seen shining through them, and they frequently cover the whole sky...
and stratocumulus clouds all have a smooth gray appearance. There are a number of features allowing the observer lots of to distinguish nimbostratus from other clouds:
- Stratus cloud
A stratus cloud is a cloud belonging to a class characterized by horizontal layering with a uniform base, as opposed to convective clouds that are as tall or taller than wide . More specifically, the term stratus is used to describe flat, hazy, featureless clouds of low altitude varying in color...
s bring much lighter precipitation (drizzle) than nimbostratus;
- Altostratus cloud
Altostratus is a cloud belonging to a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. The sun can be seen shining through them, and they frequently cover the whole sky...
s are lighter in color and less opaque than nimbostratus, so sunlight can be seen through them;
- Cirrostratus cloud
Cirrostratus clouds are thin, generally uniform clouds, composed of ice-crystals, capable of forming halos. They are usually located above 5.5 km . When thick enough to be seen, they are whitish, usually with no distinguishing features...
s never bring precipitation and have a thin, whitish, veil-like structure, characteristic of cirrus;
- Stratocumulus bring only light precipitation and have clearly visible base
This article refers to meteorology, for the airborne base of Captain Scarlet see Cloudbase.The cloud base is the lowest altitude of the visible portion of the cloud...
with easily distinguished separate cloud elements;
- Large and low cumulonimbus cloud
Cumulonimbus is a type of cloud that is tall, dense, and involved in thunderstorms and other intense weather. It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold front in a squall line. They create lightning through the heart of the cloud...
s covering most of the sky can be mistaken for nimbostratus. However, they bring heavier, less constant precipitation.
Formation
Nimbostratus will occur along warm fronts where the slowly rising warm air mass creates nimbostratus and stratus clouds, which are preceded by higher-level clouds such as cirrostratus and altostratus. Often, when an altostratus cloud thickens and descends into lower altitudes, it will become nimbostratus.
Forecast
Usually, nimbostratus is a sign of steady moderate to heavy precipitation, as opposed to the shorter period of typically heavier precipitation released by a cumulonimbus cloud. However, precipitation does not occur at ground level in case of
virgaIn meteorology, virga is an observable streak or shaft of precipitation that falls from a cloud but evaporates before reaching the ground. At high altitudes the precipitation falls mainly as ice crystals before melting and finally evaporating; this is usually due to compressional heating, because...
and accompanies other cloud types. Precipitation may last for several days, depending on the speed of the
occluded frontAn occluded front is formed during the process of cyclogenesis when a cold front overtakes a warm front. When this occurs, the warm air is separated from the cyclone center at the Earth's surface...
it accompanies.
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